1. Field of the Invention
This invention relates to an exhaust gas recirculation system for use in an internal combustion engine, and more particularly to an exhaust gas recirculation system for use in an internal combustion engine equipped with an intake-mixture-charge heating means.
2. Description of the Prior Art
In general, a riser portion is provided on an outer wall of a collecting portion of an intake manifold of an internal combustion engine, and exhaust gases or warmed cooling water is introduced into the riser portion for heating the intake manifold, thereby preventing fuel droplets from clinging to the inner wall of the intake manifold.
On the other hand, the exhaust gas recirculation system recirculates part of exhaust gases into an intake mixture system of the engine for suppressing the generation of nitrogen oxides. In this respect, recirculation of exhaust gases exerts direct influence on the operational performance of an engine, so that the recirculation should be controlled so as not to impair desired drivability of a motor vehicle powered by the engine. To meet this requirement, it has been proposed to provide a temperature-responsive valve on a water jacket in a prior art exhaust gas recirculation system, and the flow rate of exhaust gases to be recirculated is controlled in association with the temperature of the cooling water. However, with such a prior art exhaust gas recirculation system, exhaust gases are recirculated, despite the fact that the engine may remain in an instable condition, so that the operational performance of an engine is adversely affected. This is particularly true when starting a cold engine.